These quarters featured iconic images that have special meaning to each state. Not surprisingly, many of the engravings depict landscapes of national parks. The three West Coast states are especially meaningful because they have geologic features that, when the quarters are stacked, represent the surface and subsurface of the chain of volcanoes that has been developing over the past million years.
This can be demonstrated by stacking the three quarters with California on the bottom, overlain by Oregon and then Washington. Left image Mount Rainier. Credit: Photo Courtesy of Robert J. Right image Feature labels. Click on arrows and slide left and right to see labels. Rainier in Mt. Rainier National Park is an active composite volcano rising more than 14, feet above sea level. Left image Feature labels. Right image Crater Lake. Credit: Photo courtesy of Robert J.
Crater Lake in Crater Lake National Park partially fills the large depression formed when a composite volcano erupted and collapsed in on itself 7, years ago.
Left image Yosemite Valley and Half Dome. Half Dome in Yosemite National Park is made of granite that solidified from magma tens of millions of years ago, when a subduction zone extended along the entire west coast. The overlying volcanoes have since eroded away, exposing a vast expanse of the ancient magma chambers in the Sierra Nevada.
Stack the quarters with Washington on top, Oregon in the middle and California on the bottom. Looking first at Washington, one sees Mt. Rainier, a composite volcano rising to over 14, feet above sea level in Mt.
Rainier National Park. Imagine if Mt. Rainier were to suddenly and violently erupt—so much that its magma chamber empties and the volcanic peak collapses in on itself. An ancient volcano, Mt. Mazama, erupted and collapsed 7, years ago, forming the large cavity caldera that now holds Crater Lake.
Now suppose the subduction processes that form the volcanoes were to stop, shutting off the magma supply that feeds the volcanoes. With continued uplift and erosion, most of the volcanic material would erode. Remove the Oregon quarter to reveal cooled magma chamber rocks below. This is what has happened in the Sierra Nevada of central and southern California, as represented by the granite-type rocks of Half Dome within Yosemite National Park shown on the California state quarter.
Five park sites in southern Alaska showcase landscapes formed by the subduction of the northward-moving Pacific Plate beneath North America. One of them, Kenai Fjords National Park, lies within the accretionary wedge of uplifting oceanic sedimentary strata and hard crust.
Another, Wrangell-St. Elias National Park, is a vast region that extends across the accretionary wedge and volcanic arc, and is built of accreted terranes. Kenai Fjords National Park lies within a coastal mountain range accretionary wedge formed as the Pacific Plate subducts beneath southern Alaska.
Pillow basalts attest to the oceanic origins of the rock layers, as they formed from lava flows that cooled on the ocean floor. Sandstone and shale layers at Kenai Fjords are commonly metamorphosed and were so deformed during subduction and uplift that they are vertical in places.
The park is right above the focus of the Great Alaska Earthquake of , one of the two largest ever recorded the other, in , occurred along the subduction zone off western South America. Effects of the earthquake are quite spectacular in the park; the coastline dropped so much that in places it lies submerged beneath about 8 feet 2.
Kenai Fjords National Park Layers of sandstone and shale deposited on the top of the Pacific Plate were later metamorphosed to quartzite and slate as the plate subducted beneath southern Alaska, and then shoved upward as part of the accretionary wedge. Kenai Fjords National Park Sea lions resting on layers of pillow basalt that formed beneath the ocean and were uplifted as part of the accretionary wedge.
More than 40 volcanoes have erupted in Alaska since Russians first arrived in the mid s — a great many of the volcanoes are in areas that are now National Park Service sites. They are part of the volcanic arc that extends northeastward from the Aleutian Islands, across the Alaska Peninsula and Aniakchak National Monument, Katmai and Lake Clark national parks, and then bends eastward to Wrangell-St. Elias National Park.
National Park Service Sites in the Southern Alaska Subduction Zone reveal a variety of active volcanic features above the zone where the top of the Pacific Plate reaches sufficient depth where temperature and pressure are high enough to dehydrate rocks and form magma. Elias National Park Mt. Wrangell left and Mt. Drum right. The eruption of Aniakchak Volcano about 3, years ago rivals the eruption and collapse of Mt. Mazama that formed Crater Lake.
The original composite volcano was about 7, feet 2, meters high, but lost about 2, feet meters of its height. The 6-mile kilometer wide caldera is about 2, feet meters deep. Like Mt. Mazama, Aniakchak spewed lava across its caldera floor for centuries after its collapse. The volcano is still active, having erupted in But unlike Mt.
Mazama, Aniakchak no longer holds a large lake because the caldera wall is breached. The collapse caldera from the Katmai eruption is about 3 miles 5 kilometers across and 2, feet meters deep. The caldera has partially filled with water — an approximately half-scale version of the better-known Crater Lake in Oregon. Katmai volcano did collapse, but unlike Mt. Mazama 7, years before, the material expelled from its magma chamber did not spew forth out of the top of the volcano. Rather, it flowed underground to a vent 6 miles 10 kilometers away!
The vent was the source for most of the ash and pyroclastic flows expelled during the eruptions, and has since filled with a lava dome known as Novarupta.
At 20, square miles 53, square kilometers , Wrangell — St. Elias National Park and Preserve is the largest U. The park is a complex amalgamation of blocks of continental and oceanic crust that have slammed into North America. Poking through the mass of some of the highest mountains in North America are very young volcanoes formed by the ongoing Pacific Plate subduction. Among them are Mt. Wrangell, a 14, foot 4, meter volcano that last erupted during the early part of the 20th Century.
Lake Clark National Park and Preserve contains an active composite volcano with a profile and recent history reminiscent of Mt. In and , Mt. Redoubt, on the eastern side of the park, had four explosive eruptions that resulted in large volumes of ash and mudflows. And, as at Mt. The eruption of Mt. Redoubt, an active volcano in Lake Clark National Park, illustrates how a volcano can pose an unexpected hazard in our modern age — and why monitoring volcanic activity is so important see photo below.
On Dec. Without warning the plane flew right through the eruption cloud at 25, feet 7, meters altitude. Silica-rich ash melted and coated the hot engine turbines with glass, causing all four of them to shut down. After a terrifying free-fall that lasted 8 minutes, the pilots finally managed to restart the engines at only 6, feet 2, meters.
Fortunately, no one was injured. But the incident highlighted the importance of monitoring volcanic activity and making the information immediately available to the airline industry and other segments of the public. Plate Tectonics and Our National Parks.
Divergent Plate Boundaries. Convergent Plate Boundaries. Transform Plate Boundaries. Oceanic Hotspots. It is released when the oceanic crust is heats and then rises and mixes with the overlying mantle. The magma, which is lighter than the surrounding mantle material, rises through the mantle and the overlying oceanic crust to the ocean floor where it creates a chain of volcanic islands known as an island arc.
A mature island arc develops into a chain of relatively large islands such as Japan or Indonesia as more and more volcanic material is extruded and sedimentary rocks accumulate around the islands. As described above in the context of Benioff zones Figure The largest earthquakes occur near the surface where the subducting plate is still cold and strong. At an ocean-continent convergent boundary, the oceanic plate is pushed under the continental plate in the same manner as at an ocean-ocean boundary.
Sediment that has accumulated on the continental slope is thrust up into an accretionary wedge, and compression leads to thrusting within the continental plate Figure The mafic magma produced adjacent to the subduction zone rises to the base of the continental crust and leads to partial melting of the crustal rock.
The resulting magma ascends through the crust, producing a mountain chain with many volcanoes. A continent-continent collision occurs when a continent or large island that has been moved along with subducting oceanic crust collides with another continent Figure The colliding continental material will not be subducted because it is too light i.
There is tremendous deformation of the pre-existing continental rocks, and creation of mountains from that rock, from any sediments that had accumulated along the shores i. Examples of continent-continent convergent boundaries are the collision of the India Plate with the Eurasian Plate, creating the Himalaya Mountains, and the collision of the African Plate with the Eurasian Plate, creating the series of ranges extending from the Alps in Europe to the Zagros Mountains in Iran.
The Rocky Mountains in B. Transform boundaries exist where one plate slides past another without production or destruction of crustal material. As explained above, most transform faults connect segments of mid-ocean ridges and are thus ocean-ocean plate boundaries Figure Some transform faults connect continental parts of plates.
Transform faults do not just connect divergent boundaries. For example, the Queen Charlotte Fault connects the north end of the Juan de Fuca Ridge, starting at the north end of Vancouver Island, to the Aleutian subduction zone.
The boundary between the two plates is the Nootka Fault, which is the location of frequent small-to-medium earthquakes roughly up to magnitude 5 , as depicted by the red stars. Explain why the Nootka Fault is a transform fault, and show the relative sense of motion along the fault with two small arrows.
See Appendix 3 for Exercise As originally described by Wegener in , the present continents were once all part of a supercontinent, which he termed Pangea meaning all land.
More recent studies of continental matchups and the magnetic ages of ocean-floor rocks have enabled us to reconstruct the history of the break-up of Pangea. During the same period, the Atlantic Ocean began to open up between northern Africa and North America, and India broke away from Antarctica. To see the timing of these processes for yourself, go to time lapse of Continental Movements.
Within the past few million years, rifting has taken place in the Gulf of Aden and the Red Sea, and also within the Gulf of California. Over the next 50 million years, it is likely that there will be full development of the east African rift and creation of new ocean floor. Eventually Africa will split apart. There will also be continued northerly movement of Australia and Indonesia. The western part of California including Los Angeles and part of San Francisco will split away from the rest of North America, and eventually sail right by the west coast of Vancouver Island, en route to Alaska.
Because the oceanic crust formed by spreading on the mid-Atlantic ridge is not currently being subducted except in the Caribbean , the Atlantic Ocean is slowly getting bigger, and the Pacific Ocean is getting smaller.
If this continues without changing for another couple hundred million years, we will be back to where we started, with one supercontinent. This E-mail is already registered as a Premium Member with us. Kindly login to access the content at no cost. This E-mail is already registered with us. Not my Question Bookmark. Flag Content. Question : Where subduction occurring this map of South America? Wh :
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